Implementing laser eye safety for 850 nm semiconductor lasers poses a challenge. Semiconductor vertical cavity surface emitting lasers (“VCSELs”) typically operate better—and with higher bandwidth—when biased at higher currents resulting in higher output powers. Often, the Class 1 Eye Safety threshold power rating is very close to the nominal operating power of semiconductor based transceivers. From a manufactures perspective, products bearing a Class 1 Eye Safety label have a competitive advantage over lesser rated products (e.g., Class 1 M Eye Safety label) in the marketplace.
One conventional approach limits power output to achieve the Class 1 Eye Safety rating by selecting a direct current (“DC”) bias current value with sufficient safety margin such that an entire population of devices is guaranteed not to exceed output power safety limits designated by the Class 1 Eye Safety standard. As operating temperature of an optical source changes, this maximum DC bias current changes. Additionally, as semiconductor laser sources age, the maximum DC bias current for a given temperature also changes. Accordingly, the selection of an appropriate DC bias current value with sufficient safety margin for an entire population of laser sources must account for temperature and aging factors. The safety margins built-in to a selected DC bias current value necessarily compromises eye mask margin for adherence to eye safety standards. Eye mask margin is a measure of the fidelity of an optical signal and is directly correlated with signal bandwidth. FIG. 1 illustrates how eye mask margin increases with laser bias current.
Another conventional approach limits output power for adherence to eye safety standards using a current sensing resistor coupled in series with a laser driver of the laser source. This series coupled current sensing resistor feeds a comparator coupled to a reference level. When a signal from the current sensing resistor exceeds the reference level, the laser source is disabled or current limited. A variation of this technique includes a digital potentiometer to vary a trip threshold with temperature. However, in either case, the protection mechanism is based on laser bias current, which is not truly representative of output power—the factor by which Laser Eye Safety is measured. Laser bias current is not an accurate indicator of output power since the relationship between output power and bias current varies with age.
To adhere to Laser Eye Safety standards, while operating at optimum bandwidth, a semiconductor laser source should continuously operate as close as possible to the maximum allowable power output. To safely operate continuously near the power output safety limit, robust safety mechanisms should be in place to detect and act on eye safety violation conditions.